1. Field of the Invention
The present invention relates to a varnish and a prepreg, in particular to a varnish containing with sericite powders, a prepreg made from the varnish, and a substrate made from the prepreg or prepregs.
2. Description of Related Art
A printed Circuit Board (PCB) is made by laminating pressing and curing the laminated prepregs, copper clad laminate (CCL), or copper film. The prepregs are made, generally, by dipping glass fabrics into vanish and then curing the fabrics to form thin prepregs.
A PCB is made by pressing the prepregs, copper clad laminate (CCL), or copper film. The prepregs are made by dipping glass fabrics into varnish and then curing the fabrics to form thin prepregs. In seeking enhancement of environmental protection, lead-free solder is more and more widely used for assembly in electronic applications. However, this necessitates that the process temperature must be increased by about 30 to 40° C. for the lead-free solder processes and it is necessary to improve the heatproof or heat-resistance properties of the PCBs so the PCBs can be used in higher temperature process.
Traditionally, the originally-used DICY (dicyandiamide) curing agent with polarity and higher water absorption is replaced for the phenol curing agent with a benzene ring structure. Furthermore, inorganic silico-powders are usually added into the varnish to improve the heat-resistance properties of the PCBs. However, the inorganic silico-powders have high hardness and the manufactured prepregs/substrates are too hard and brittle to be machined. For example, while drilling on the hard and brittle substrates, the surfaces of the drilled holes are rough, or the laminated layers of the substrate are de-laminated. There are often cracks inside the substrate. In these circumstances, the drill bit is easily broken and the error rate and manufacturing cost is increased.
In other words, the harder silico-powders, such as SiO2 powders with Mohs' scale of 7, are added into the varnish and the hard fillers cause low machining capabilities in the manufactured prepregs/substrates.
Therefore, in view of these difficulties, the inventor has developed ways to overcome these difficulties to yield dependable product with reliable results in production; the present invention addresses these difficulties and allows reliable production at high speed and overcomes the above problems.